U.S. patent number 6,667,839 [Application Number 10/033,670] was granted by the patent office on 2003-12-23 for holding device for an optical element made of a crystalline material.
This patent grant is currently assigned to Carl Zeiss SMT AG. Invention is credited to Harry Bauer, Thure Boehm, Michael Gerhard, Jurgen Hartmaier, Dietrich Klaassen, Ralf Kuschnereit, Jens Spanuth, Peter Vogt, Bernhard Wergl.
United States Patent |
6,667,839 |
Hartmaier , et al. |
December 23, 2003 |
Holding device for an optical element made of a crystalline
material
Abstract
A device is used to hold an optical element, in particular one
made of a crystalline material, in particular of CaF.sub.2, while
the optical element is being coated, in particular by the
vapor-deposition of at least one functional layer in a vacuum
coating plant. The latter has a device for mounting the optical
element, it being possible for the optical element to be heated in
the vacuum coating plant via suitable radiation, in particular
infrared radiation. An intermediate element which has a lower
thermal absorption than the device for mounting the optical element
is arranged between the device for mounting the optical element and
the optical element.
Inventors: |
Hartmaier; Jurgen (Aalen,
DE), Klaassen; Dietrich (Oberkochen, DE),
Boehm; Thure (Aalen, DE), Wergl; Bernhard
(Steinheim, DE), Gerhard; Michael (Aalen-Ebnat,
DE), Spanuth; Jens (Aalen, DE),
Kuschnereit; Ralf (Oberkochen, DE), Vogt; Peter
(Heidenheim, DE), Bauer; Harry (Aalen,
DE) |
Assignee: |
Carl Zeiss SMT AG
(DE)
|
Family
ID: |
7669832 |
Appl.
No.: |
10/033,670 |
Filed: |
December 27, 2001 |
Foreign Application Priority Data
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Jan 5, 2001 [DE] |
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101 00 328 |
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Current U.S.
Class: |
359/820; 353/100;
359/811; 359/819; 362/455; 396/526 |
Current CPC
Class: |
C23C
14/50 (20130101); C23C 14/505 (20130101) |
Current International
Class: |
C23C
14/50 (20060101); G02B 007/02 (); G03B 017/26 ();
G03B 021/14 (); F21V 017/00 () |
Field of
Search: |
;359/819,811,820
;396/526 ;362/455 ;353/100 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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679 838 |
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Nov 1990 |
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CH |
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44 46 179 |
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Dec 1994 |
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DE |
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199 32 338 |
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Jul 1999 |
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DE |
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0 215 261 |
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Aug 1986 |
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EP |
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0 743 377 |
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Nov 1996 |
|
EP |
|
08136862 |
|
May 1996 |
|
JP |
|
08194755 |
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Feb 1998 |
|
JP |
|
Primary Examiner: Dang; Hung Xuan
Assistant Examiner: Martinez; Joseph
Attorney, Agent or Firm: Wells St. John P.S.
Claims
What is claimed is:
1. A holding device for an optical element, while the optical
element is being coated in a vacuum coating plant having a
mounting-device for the optical element, it being possible for the
optical element to be heated in the vacuum coating plant via
suitable radiation, wherein an intermediate element which has a
lower thermal absorption than the mounting-device for the optical
element is arranged between the mounting-device for the optical
element and the optical element.
2. The device as claimed in claim 1, wherein the intermediate
element is connected to the mounting-device for the optical element
via supporting elements made of a poorly heat-conducting
material.
3. The device as claimed in claim 2, wherein the supporting
elements are designed as geometric bodies having a low contact
area, at least in relation to the mounting-device for the optical
element.
4. The device as claimed in claim 2, wherein the supporting
elements are designed as cylindrical elements.
5. The device as claimed in claim 2, wherein the supporting
elements are designed as spherical elements.
6. The device as claimed in claim 2, wherein the supporting
elements are designed as conical elements.
7. The device as claimed in claim 2, wherein the supporting
elements are formed from a material with a low thermal
conductivity.
8. The device as claimed in claim 2, wherein the supporting
elements are formed from ruby.
9. The device as claimed in claim 2, wherein the supporting
elements are formed from ceramic.
10. The device as claimed in claim 1, wherein the intermediate
element is formed of a material which absorbs the suitable
radiation to a lesser extent than the mounting-device for the
optical element.
11. The device as claimed in claim 1, wherein the intermediate
element has a coating that reflects the suitable radiation.
12. The device as claimed in claim 1, wherein the intermediate
element is formed from a material which is softer than the material
of the optical element.
13. The device as claimed in claim 11, wherein the reflective
coating has gold.
14. The device as claimed in claim 1, wherein the intermediate
element is formed from an alloy containing aluminum.
15. The device as claimed in claim 1, wherein the optical element
is made of a crystalline material.
16. The device as claimed in claim 1, wherein the optical element
is made of CaF.sub.2.
17. The device as claimed in claim 1, wherein the optical element
is being coated by a vapor-deposition of at least one functional
layer.
18. The device as claimed in claim 1, wherein the mounting-device
comprises high-strength material.
19. The device as claimed in claim 1, wherein the mounting-device
comprises steel.
Description
RELATED APPLICATIONS
This application relates to and claims priority to corresponding
German Patent Application No. 101 00 328.5 filed on Jan. 5,
2001.
BACKGROUND OF THE INVENTION
The invention relates to a holding device for an optical element,
while the optical element is being coated in a vacuum coating plant
having a mounting-device for the optical element, it being possible
for the optical element to be heated in the vacuum coating plant
via suitable radiation.
In general, optical elements are very frequently coated with
functional layers to improve the optical quality, for example
antireflection coatings or the like. This coating is normally
carried out in a vacuum coating plant, into which the optical
elements are introduced and in which they are heated while, at the
same time, the substances for the corresponding functional layer
are fed in. The substances for the corresponding functional layer
are then deposited on the surfaces of the optical elements. In
order to achieve the most uniform deposition possible on the
surfaces of the optical elements, the latter are inserted into
corresponding mountings for holding the optical elements and are
generally moved in the vacuum coating plant.
Since, then, lenses of this type or other optical elements made of
crystalline materials, which are used for example in the
semiconductor lithography technique, have relatively large
diameters and are comparatively heavy, these devices for mounting
the optical elements in the vacuum coating plant, with the
corresponding driven axes, are designed from high-strength
materials, in order for example to be able to coat four or more
lenses simultaneously in the vacuum coating plant, without the
mountings failing as a result of the weight loading together with
the thermal loading which occurs. For this purpose, the holding
elements are generally designed from a high-strength metal, for
example steel.
Then, because of the vacuum or at least approximately complete
vacuum, appropriate radiators must be used in order to ensure that
the optical elements are heated up, since the transfer of heat as a
result of convection or the like cannot be utilized in a vacuum. In
the case of these radiators, these are conventionally infrared
radiators, but there is the problem that these also heat the
components which surround the optical elements and which, as
already mentioned at the beginning, are usually designed from steel
to a very great extent. In this way, a very large temperature
difference arises between the devices for mounting the optical
element and the optical element itself, which leads to a
temperature gradient within the optical element.
Therefore, in the area in which the optical element rests on the
mountings, a very large point input of heat occurs, since here the
mountings, which generally absorb the heat better than the optical
element, introduce a very large amount of thermal energy into the
optical element, said energy being transported away only
inadequately by the latter, since crystalline materials of the type
mentioned at the beginning are generally poorer heat conductors
than steel, and it being possible for said energy to be distributed
in the element. A very high temperature gradient is therefore
established in the optical element itself as well, which leads to
high thermal stresses in the optical element. This increase in the
thermal stress normally takes place in an area in which, as a
result of the fact that the optical element is supported on the
devices in order to mount it, very high inputs of stress in any
case act on the optical element, because of the forces of gravity.
It is therefore very easy for a critical shear stress to be
exceeded in these areas of the optical element, as a result of the
addition of gravitational stress and thermally induced stress. In
the case of crystalline materials, this can lead to an offset in
the lattice planes or the like, which makes the optical elements
unusable for further applications in the area of high-power optics,
since plastic deformation occurs.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide a device
which is used to hold an optical element, in particular one made of
a crystalline material, during coating of the optical element, in
particular by means of vapor deposition of at least one functional
layer in a vacuum coating plant, and which avoids the input of
thermal loadings into the optical element.
According to the invention, this object is achieved by the features
recited in claim 1.
The fact that an intermediate element is used, which has a lower
heat absorption than the device for mounting the optical element,
means that the thermal energy present in the mountings does not
reach the optical element to the full extent. The input of thermal
energy into these areas, which are in any case very critical, of
the optical element in the area in which it is supported is
avoided, and the addition of thermally induced stresses and
stresses in the crystal of the optical element, introduced by
gravitation, therefore generally remains under a critical shear
stress which could effect plastic deformation of the optical
element.
In a particularly beneficial refinement of the invention, the
intermediate element is additionally connected to the device for
mounting the optical element via supporting elements made of a
poorly heat-conducting material.
This provides a further advantage. Here, the input of thermal
energy from the mountings to the intermediate element, and
therefore also to the optical element, can be virtually completely
prevented, since the corresponding supporting elements, which can,
for example, be designed as small spheres made of ceramic or the
like, virtually do not pass on the heat into the intermediate
element and therefore into the optical element, in particular also
because of their small contact area.
In a further, very beneficial embodiment of the invention, the
intermediate element additionally has a coating that reflects the
radiation used to heat the optical element.
This coating, which may be composed of gold or the like, virtually
completely reflects the radiation used to heat the optical element,
so that the intermediate element which, in a particularly
beneficial combination of the two embodiments described, is
additionally thermally decoupled from the mountings via the
supporting elements, is virtually not heated or, in any case, no
more than the optical element itself. The input of thermally
induced stresses into the optical element, which are caused by
local heating and associated high temperature differences between
the individual points of the optical element, can therefore be
eliminated virtually completely. Coating the optical element with
one or more appropriate functional layers is then possible without
difficulty, without any impairment of the optical quality of the
optical element on account of mutually offset lattice planes or the
like, that is to say plastic deformation, having to be feared.
A further advantage is that the intermediate element can be
designed in such a way that it has a very good thermal conductivity
and, at the same time, a very low heat capacity. It is then
possible for the intermediate element virtually always to be at the
same temperature as the optical element, since thermal differences
are balanced out very quickly as a result of its good thermal
conductivity. As a result of the simultaneously very small heat
capacity, which can be achieved for example by means of a
relatively thin design of the intermediate element, with a very low
mass, barely any storage effects, which delay the temperature
differences over time, occur in the area of the intermediate
element.
In principle, intermediate elements made of corresponding,
temperature-resistant plastics would of course also be conceivable
here, since these would combine very advantageous properties with
regard to heat capacity and heat conduction with very beneficial
mechanical properties. However, it should then be ensured that, in
the vacuum coating plant, no organic substances can evaporate off
from the plastics, which might be deposited on the optical element
and could cause impairment of the quality to be achieved of a
functional coating on the optical element.
BRIEF DESCRIPTION OF THE DRAWINGS
Further advantageous refinements emerge from the remaining
subclaims and from the following exemplary embodiment, using the
drawings.
In the drawings:
FIG. 1 shows a highly schematic vacuum coating plant in a basic
cross section; and
FIG. 2 shows a cross section through part of a device for mounting
the optical element to be coated, according to the line II-II in
FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 indicates a vacuum coating plant 1 in a very highly
schematic representation. In the interior of the vacuum coating
plant 1, two devices 2 for mounting one optical element 3 each are
represented in basic form. The devices 2 for mounting the optical
element 3 are in this case each connected via a holding mechanism 4
to a shaft 5, each of which rotates in accordance with the arrows
A. This unit comprising the holding mechanism 4, shaft 5 and device
2 for mounting the optical element 3 is then fixed to a crossbeam
6, which in turn rotates about an axle 7 in accordance with the
arrow B. The individual optical elements 3 therefore execute
cycloidal movements by which means uniform coating of the surface
of the optical elements can be achieved. It is usual for more than
two of the optical elements to be fixed to the crossbeam 6 via the
corresponding components 5, 4, 2, so that four or more optical
elements 3 can be provided simultaneously in the vacuum coating
plant 1 with a surface layer, for example an antireflection
coating.
In order to be able to heat up the optical elements 3 for the
evaporation with the corresponding functional layer, in spite of
the vacuum in the vacuum coating plant 1, the vacuum coating plant
1 has a plurality of radiators 8, which are indicated here in
principle. By means of the radiation emerging from the radiators 8,
the optical elements 3 are heated up to the temperature required
for the evaporation. The radiators 8 are usually infrared radiators
but, depending on the optical elements 3 to be coated, other
wavelengths of radiation are also conceivable which in each case
lead to the heating of the optical element 3.
At the same time of course, the radiators 3, via their radiation,
also heat up the structure comprising the devices 2 for mounting
the optical elements 3, the holding mechanism 4, the shafts 5, the
crossbeam 6 and the axle 7. Because of the thermal and mechanical
loadings on these elements, these are normally designed from a
high-strength material, for example steel, and heat up to a much
greater extent than the actual optical element 3 to be heated.
In the case of the previous plants, a large temperature difference
therefore occurs between the devices 2 for mounting the optical
element 3 and the optical element 3 itself. In the region in which
the optical element 3 is supported on the devices 2, thermal
conduction produces high point heating of the optical element 3,
and said heating can propagate only inadequately in the latter,
because of its generally comparatively poor thermal conductivity.
The result is very high stresses because of the temperature
gradient which additionally lie in the range in which, because the
optical element 3 is supported on the devices 2 for mounting it,
they already effect very high inputs of stress as a result of the
gravitational force on the optical element 3.
If, then, high-value optical elements 3 made of crystalline
materials, preferably of fluorides, for example calcium fluoride,
are coated, the coating of other single crystals, for example made
of germanium or the like, is also conceivable here, however, so
that because of the superimposition of thermally induced stresses
and stresses caused by gravitation, it is possible for a very high
shear stress to occur in the critical area of the support of the
optical element 3. As a result of this high shear stress,
individual lattice planes of the crystalline optical element 3 can
be displaced with respect to one another, as a result of which the
optical element 3 becomes unusable for its planned intended use,
since it is then plastically deformed.
In the case of the device illustrated here, this is avoided by the
intermediate element 9 which can be seen in FIG. 2, and also by
corresponding supporting elements 10. For this purpose, the
intermediate element 9 is formed from a material which absorbs the
radiation used for heating from the radiators 8, for example
infrared radiators, to a lesser extent than the optical element 3
itself. This material can be, for example, an appropriate material
made reflective. An intermediate element 9 made of aluminum and
provided with a reflective gold coating has proven to be
particularly beneficial. The aluminum, made reflective with the
gold, reflects the greatest part of the incident infrared radiation
from the radiators 8 and therefore, as opposed to the surrounding
devices 2, is heated to a far lesser extent. The input of thermal
energy into the edge region of the optical element 3 which is in
any case highly loaded by the gravitational stresses, by the
intermediate element 9 and by the devices 2 for mounting the
optical element 3, can therefore be avoided.
In order, then, to decouple the intermediate element 9 thermally
from the device 2 for mounting the optical element 3, the
intermediate element 9 is supported on the device 2 via the
supporting elements 10. In this case, the supporting elements 10
are designed from a material which conducts the heat to a far
poorer extent than the device 2, normally designed from metal,
and/or the intermediate element 9.
In addition, the supporting elements 10 are designed in a geometric
form which forms only a very low contact area between the
supporting element 10 itself and the device 2, on the one hand, and
the intermediate element 9 on the other hand. Here, for example,
thought can be given to conical, cylindrical or spherical elements.
If the supporting elements 10 are at the same time additionally
formed from a material with a correspondingly poor heat conduction,
for example from a ceramic, then the input of heat into the optical
element 3 by heat conduction from the device 2 for mounting the
optical element 3 via the supporting elements 10 and the
intermediate element 9 can virtually be ruled out. However, in
particular because of their ideal mechanical properties, spheres
made of ruby have been shown to be particularly suitable as
supporting elements 10. Although these have a better thermal
conductivity than ceramics, because of the very small contact area
of the spheres, the thermal decoupling also functions very well
here. The abovementioned problems relating to exceeding the
critical shear stress because of the addition of thermal stresses
and gravitational stresses in the area in which the optical element
3 is supported can therefore be avoided.
If the intermediate element 9, as described above, is an aluminum
ring provided with a reflective gold coating, then this is
additionally softer than the material normally used for the optical
element 3, for example calcium fluoride, so that the material of
the optical element 3 does not suffer any damage due to scratches
or the like which, in the case of crystalline materials of this
type could very easily lead to a lattice offset, cleavage or the
like.
Of course, the intermediate element 9 is likewise vapor-coated with
the layer applied to the optical element 3 by the vacuum coating
plant 1 and, in the case of an antireflection coating, following
repeated use leads to it no longer being possible completely to
ensure the reflection of the radiation of the radiators 8 used for
heating. The intermediate elements 9 then have to be cleaned or
replaced.
On account of these considerations, the thermally resistant plastic
already mentioned hereabove would be a very beneficial material for
the intermediate ring 9, since this would be relatively soft and,
on account of the comparatively small expenditure during its
production, could be designed as a disposable article.
* * * * *